Conventionally, a battery pack having a rechargeable battery provided with a temperature detecting unit for detecting a temperature of the rechargeable battery has been used in a power tool or the like. The battery pack is independently formed of a tool's body and can be separated from the tool's body to be mounted on a charging device during the charging operation. As shown in FIG. 10, as the charging operation comes to an end thereof, a temperature increasing rate (ΔT/Δt) of the rechargeable battery detected by the temperature detecting unit increases substantially. Based on this principle, and responsive to a detection result of the temperature detecting unit, a controller determines that a full capacity charge of the rechargeable battery has been reached when the temperature increasing rate (ΔT/Δt) is equal to or greater than a first predetermined value and stops the charging operation accordingly.
It is known that a carrying out of the charging operation of the battery pact at a low temperature may result in shortening of the life time thereof. In view of this, there is known a charging device of a type that, at a low temperature of, e.g., not greater than 0° C., even though a battery pack is set on a charging device, the charging device does not carry out a normal charging operation and, instead, enters into a low temperature standby mode in which the battery pack is supplied with a minimal or no electric current. Also known is a charging device of a type that, even when a charging operation is in progress, the charging operation is switched into a low temperature standby mode if the temperature of the battery reaches a low temperature as noted above. Thereafter, the charging operation is restarted when the temperature of the rechargeable battery reaches, e.g., 5° C.
Even in the battery pack protected with the low temperature standby mode, however, a normal mode charging operation proceeds as the temperature to which the battery pack is exposed is increased. For this reason, in the event that the battery pack, which was left alone at a cold outdoor environment of equal to or colder than −10° C., is brought into a room of about 30° C. and then set on a charging device, the temperature increasing rate becomes substantial due to the difference between the outdoor temperature and the room temperature. As a consequence, notwithstanding the fact that the charging operation has not yet been completed, a misjudgment is made that the charging operation has been completed as illustrated in FIG. 11, thus resulting in a poor charging problem referred to as “premature charging cessation”.
Japanese Patent Laid-open Publication No. H6-141481 discloses a prior art for avoiding such a poor charging problem. In accordance with this prior art, a temperature increasing rate is not detected for a time period corresponding to the temperature of the rechargeable battery at the time of initiating the charging operation. This helps not only to avoid the poor charging problem but also to prevent the time for starting the detection of the temperature increasing rate from being prolonged unwantedly.
The prior art cited above is effective in avoiding the premature charging cessation and charging the rechargeable battery up to a somewhat acceptable capacity level. However, the rechargeable battery thus provided exhibits different performance depending on its condition of use and shows a marked reduction in performance at a low temperature. Although the prior art does not detects the temperature increasing rate in consideration of the time corresponding to the temperature of the rechargeable battery, there still exists a possibility that the rechargeable battery cannot be fully charged. This is because the determination of completion of the charging operation is made by a threshold value of a predetermined temperature increasing rate.